1. Field of the Invention
The present invention is directed to a process for preparing a dimensionally stable, biaxially stretched thermoplastic film, such as polyethylene terephthalate (PET) or polyethylene naphthalate (PEN), having low thermal shrinkage.
2. Description of the Prior Art
Commercial applications exist which require dimensionally stable thermoplastic films, such as polyethylene naphthalate (PEN) or polyethylene terephthalate (PET), having low thermal shrinkage. Examples include flexible printed circuitry, membrane touch switches, high temperature labels and certain types of pressure sensitive tapes. Current commercial thermoplastic film stabilization techniques, however, have disadvantages due to the added cost associated with additional processing steps and/or equipment, increased scrap generation with off-line equipment, limited flexibility of the extent of machine direction (MD) relaxation and less control of transverse direction (TD) tension and relaxation while imparting MD relaxation.
U.S. Pat. No. 3,150,433 discloses a simultaneous biaxial film stretcher wherein the tenter clips are driven mechanically by a large screw with varying pitch along the clip path, thereby allowing introduction of MD and TD relaxation during heat-setting of the film. However, the MD stretch ratio and MD relaxation are pre-determined by the screw pitch and cannot be readily manipulated to impart varying MD relaxation.
U.S. Pat. No. 5,429,785 discloses a process for preparing an ultra-thin, simultaneous biaxially stretched polyester film in a tenter frame using linear electric motors by heating the film to the film orientation temperature in the tenter frame prior to stretching and maintaining the film within the film orientation temperature range using radiant heaters while simultaneously stretching and subsequently heat-setting the film in the tenter frame at elevated temperatures. The patent is silent regarding relaxation of the film in the MD and TD directions to lower thermal shrinkages.
Other in-line stabilization techniques include relaxing the film by passing it through a nip formed by two rolls (at or above the glass transition temperature) which operate at different speeds and by heating under low tension in an air flotation oven. Since the film is not restrained in the tenter clips during these relaxation processes, there is less control of transverse direction tension during relaxation. In addition, both of these methods require additional processing equipment which increases cost.
Additional film stabilization units are also available as off-line processing units. While films with low thermal shrinkages can be obtained, this introduces the added cost of an additional processing step. Also, during the start-up and shut-down of this equipment, scrap film is generated thereby lowering production yields.
Accordingly, there is a need for a process for preparing biaxially stretched thermoplastic films, such as polyethylene naphthalate or polyethylene terephthalate film, which provides greater flexibility and control of the amount of MD and TD film relaxation thereby producing films with low MD and TD thermal shrinkages.